1. Field of the Invention
The invention pertains generally to the field of methods for minimizing unwanted system dynamics. More particularly, the invention pertains to a method and system for vibration avoidance for use with automated machinery.
2. Description of Related Art
Automated machines often have performance limitations caused by the vibration within the mechanisms and the stationary structure to which these mechanisms are attached within the machinery. Often, no matter how much energy is applied to a mechanism it is unable to perform the assigned task faster than a certain rate because vibrations interfere with the successful completion of the task. In these machines, although the application of more energy typically allows the mechanism to operate faster, the increased energy causes the amplitude of the vibrations to increase. The increased vibrations interfere with the machine's operation, especially the timely and precise stopping of an apparatus at a target point or value.
In the past, these problems have been dealt with using engineering methods, e.g., carefully designing the machine elements to not vibrate by using special materials, increasing the mass and/or stiffness of the frame and carefully designing the overall structure of the apparatus to avoid vibration, etc. These methods can only solve the problem to a limited degree, tend to increase the assembled cost of the machine, and require additional design effort and engineering cost to implement. Sometimes the successful application of these mechanical solutions is not possible because of constraints imposed by the very task the machine is attempting to perform.
Every contour that machine elements are asked to follow as a function of time is referred to herein as a space-time contour where “space” is the dependent physical variable to be controlled (i.e., space variable) as a function of the independent variable time. Typical physical variables to be controlled might be the height of a car, the pressure of a vessel, temperature of a slab, speed of a rotating disk, angle of an arm, etc. The physical variable to be controlled is “forced” by an actuator that might be a hydraulic piston, pump, burner, etc.
To understand the problem more fully, one has to be aware that every space-time contour contains energy at multiple frequencies, and one can transform space-time contours into a function of energy versus frequency and vice versa. Typical space-time contours that machine elements are asked to follow are similar to an impulse and contain a broad spectrum of frequencies much like the impulse from a drumstick striking a surface has a broad spectrum of frequencies. If the drumstick strikes a foam pad on a board the result is a dull thud because the pad is similar to well damped machine elements without resonance so no frequencies are amplified or sustained. If the drumstick strikes a gong or a tuning fork the result is sustained vibration. Imagine then, an exaggerated example where the machine elements in a machine have a gong they are trying to move as a load, or alternately a tuning fork within a mechanical transmission. The goal then is to have the machine elements follow a space-time contour, for example, to get the gong and/or tuning fork from point A to point B while keeping the gong and/or tuning fork from vibrating at their natural frequencies (exciting frequencies). Simply put, this can be accomplished by removing the energy from the mechanical actuator that moves from point A to point B at the exciting frequencies that would excite the gong and/or the tuning fork.
In the past, control methods have been employed to remove the energy at the exciting frequencies so as to limit the vibration to help free the machine designer from vibration constraints. These control methods fall into two different classes.
The first class of control method that has been employed is to remove the frequency or frequencies that cause problematic vibration (exciting frequencies) directly from the energy source driving the mechanism (typically electrical current or hydraulic flow) through an actuator. (See, FIG. 1). This is accomplished by placing a frequency selective filter in the path of the energy source control. This method reduces the exciting frequencies entering the machine through the actuator. Feedback is usually employed from sensor(s) on a machine element(s) to modulate this power source through the filter to force the mechanism to follow its desired space-time contour. This control method has limitations in that the space-time contour command itself still likely has energy at frequencies that will cause vibration in the machine. By removing the exciting frequencies from the energy source driving the machine elements, errors are induced limiting the faithful following of the space-time contour, and often, these errors are unacceptable to the machine operation. In many instances, if the filtering of the energy source is not sufficient and/or robust, the feedback control, attempting to follow the desired space-time contour will force energy back into the system at the exciting frequencies, limiting the usefulness of this control solution.
The second class of control method that has been employed is to reduce the level of exciting frequencies that cause vibration from the space-time contour that the machine elements are to follow. (See, FIG. 2). This is accomplished by taking a space-time contour which has been designed to meet the application objectives and routing it through a frequency elimination filter that selectively removes most of the energy at the exciting frequencies to produce a “non-exciting” contour. This “non-exciting” contour is then delivered as the command to the control system that modulates the actuator driving machine elements in a machine. The contours for one or more apparatus within the machine may be filtered in this way to reduce vibrations. This method has proven to be effective to various degrees in reducing machine vibration, dependent upon the level of suppression provided by the filter and the robustness of the filter's effectiveness with changes in the machine elements. The unfortunate side effect of this technique is distortion of the “non-exciting” contour from the original desired space-time contour. In fact, specific versions of this method are often described as a method to “shape” the space-time contour. These specific methods to shape a contour are, in fact, a specialized frequency selective filter that has been designed by a specific time-domain technique, see U.S. Pat. No. 4,916,635. Space-time contour filtering produces, in any case, a contour that is distorted from the original designed space-time contour such that it usually degrades the performance of the machine, often causing it to not meet the design objectives. In the case of a space-time contour that is intended to move a mechanical load from one point to another and then stop, this method always increases the time between these points because delay, inherent in the filter operation, necessarily lengthens the “non-exciting” contour. This has the likely effect of slowing the overall machine, reducing its operating throughput.
Thus, there is a need in the industry for an improvement over these techniques to allow more freedom from constraints in the design of mechanisms within machines and/or to increase the throughput from existing mechanism designs without suffering the deleterious effects of induced vibration.